This invention relates generally to epitaxial growth techniques, and more particularly, to epitaxial growth of Group II-VI semiconductor materials.
As is known in the art, Group II-VI semiconductor epitaxial materials, such as cadmium telluride and mercury cadmium telluride have important applications as photodetector elements for detection of electromagnetic energy in the spectral range from approximately 0.8 .mu.m to 30 .mu.m. By adjusting the alloy composition, photodetector elements comprised of mercury cadmium telluride may be fabricated to cover certain wavelength ranges within the 0.8 .mu.m to 30 .mu.m band. That is, for short wavelength applications, i.e., those wavelengths close to 0.8 .mu.m, cadmium telluride (CdTe) is generally used. For longer wavelengths greater than approximately 0.8 .mu.m, mercury cadmium telluride (Hg.sub.1-x Cd.sub.x Te) is used where x is the composition ratio of cadmium to tellurium. Therefore, by adjusting the alloy composition, an epitaxial material is provided to cover portions of the entire spectral range of 0.8 .mu.m to over 30 .mu.m. That is, photodetector elements may be fabricated to be responsive to particular wavelength bands of infrared electromagnetic energy.
Several different epitaxial growth techniques have been suggested for providing epitaxial layers of cadmium telluride and mercury cadmium telluride suitable for use in photodetector applications.
One method suggested in the art for fabricating epitaxial layers of Group II-VI materials is growing such layers by liquid phase epitaxy (LPE). However, it has been found that growing such epitaxial layers with LPE growth techniques has several problems. In the LPE growth process, the substrate is brought into direct contact with a growth melt of the material which is to be epitaxially grown thereon. As a consequence of this contacting, the surface morphology of the substrate as well as the grown epitaxial layers is degraded due to back-etching (i.e., partial dissolving) of the substrate by the melt, so-called dendritic growth (formation of small crystallites which adhere to the epitaxial layer surface) and incomplete melt removal after growth. A second problem with LPE, due to the substrate melt contact, is that the substrate must be chemically similar to the melt, otherwise, the substrate may be dissolved by the melt or, alternatively, the melt may not wet the substrate thereby inhibiting the growth of an epitaxial film. This problem prevents the use of the LPE techniques for growing epitaxial layers of Group II-VI material on foreign or different, chemically dissimilar substrate materials. A further problem with the LPE technique is that in order to provide epitaxial growth of HgCdTe films, the substrate temperature must be approximately 500.degree. C. At this substrate temperature interfaces, or the interdiffusion of cadmium and mercury, between the substrate and the epitaxial layer increase, providing material having a nonuniform composition and hence incorrect infrared detection characteristics. A further problem with this technique is that the resulting material has a high concentration of electrically active impurities as well as native crystal defects which are difficult to control, therefore providing crystals having unpredictable electrical properties. Thus, prior to any device fabrication, the substrate and epitaxial layer are generally annealed.
A second method suggested to overcome some of the problems associated with liquid phase epitaxial growth is the so-called metalorganic vapor phase epitaxy (MOVPE), also referred to as metalorganic chemical vapor phase deposition (MOCVD) technique. The MOCVD technique involves directing vapors of alkyls of Group II and Group VI elements into a reactor vessel and chemically reacting the metalorganics to provide the epitaxial Group II-VI material. As described in an article entitled "The Growth of Cd.sub.x Hg.sub.1-x Te Using Organometallics" by J. B. Mullin et al, Journal of Vacuum Science Technology, Volume 21, No. 1, May-June 1982, and as described in an article entitled "Vapor Phase Epitaxy of CdHg.sub.1-x Te Using Organometallics" by J. B. Mullin et al, Journal of Applied Physics, Volume 14, 1981, pps. L149-151, while heretofore epitaxial growth of mercury cadmium telluride epitaxial films has been demonstrated, as indicated in these articles, the quality of the epitaxial films grown are not suitable for use in the photodetector applications. Such materials are not suitable because they have relatively high carrier concentrations and the electron mobilities of the material are relatively low. These two properties are important in photoconductive detector elements, because such elements act as infrared sensitive resistors having a conductivity between a pair of contacts thereof which changes in response to incident infrared energy. If the carrier concentration is too high, or the electron mobility is too low, carriers generated in response to the incident infrared radiation will recombine without the conductivity of the device changing between the terminals.